Multicast trees for virtual private local area network (LAN) service multicast

ABSTRACT

Principles of the invention are described for providing virtual private local area network service (VPLS) multicast instances across a public network by utilizing multicast trees. In particular, the VPLS multicast instances transport layer two (L2) multicast traffic, such as Ethernet packets, between customer networks via the public network. The principles described herein enable VPLS multicast instances to handle high bandwidth multicast traffic. The principles also reduce the state and the overhead of maintaining the state in the network by removing the need to perform snooping between routers within the network.

This application claims the benefit of U.S. Provisional Application No.60/605,629, filed Aug. 30, 2004, the entire content of which isincorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following applications:

“Aggregate Multicast Trees for Multicast Virtual Private Networks,” byRahul Aggarwal and Yakov Rekhter, Ser. No. 11/212,509, filed the sameday as the present application;

“Multicast Data Trees For Multicast Virtual Private Networks,” by RahulAggarwal, Yakov Rekhter and Anil Lohiya, Ser. No. 11/212,500, filed thesame day as the present application;

“Reliable Exchange Of Control Information For Multicast Virtual PrivateNetworks,” by Rahul Aggarwal, Yakov Rekhter and Anil Lohiya Ser. No.11/212,507, filed the same day as the present application;

“Transport of Control And Data Traffic For Multicast Virtual PrivateNetworks,” by Rahul Aggarwal, Yakov Rekhter and Anil Lohiya, Ser. No.11/213,636, filed the same day as the present application;

“Shared Multicast Trees For Multicast Virtual Private Networks,” byRahul Aggarwal and Yakov Rekhter, Ser. No. 11/213,638, filed the sameday as the present application;

“Label Switching Multicast Trees For Multicast Virtual PrivateNetworks,” by Rahul Aggarwal, Yakov Rekhter and Anil Lohiya, Ser. No.11/212,475, filed the same day as the present application;

“Aggregate Multicast Trees For Virtual Private Local Area Network (LAN)Service Multicast,” by Rahul Aggarwal and Yakov Rekhter, Ser. No.11/213,637, filed the same day as the present application;

“Multicast Data Trees For Virtual Private Local Area Network (LAN)Service Multicast,” by Rahul Aggarwal and Yakov Rekhter, Ser. No.11/212,490, filed the same day as the present application;

“Exchange Of Control Information for Virtual Private Local Area Network(LAN) Service Multicast,” by Rahul Aggarwal and Yakov Rekhter, Ser. No.11/213,639, filed the same day as the present application;

“Auto-Discovery Of Multicast Virtual Private Networks,” by RahulAggarwal and Yakov Rekhter, Ser. No. 11/213,640, filed the same day asthe present application; and

“Inter-Autonomous System (AS) Multicast Virtual Private Networks,” byRahul Aggarwal and Yakov Rekhter, Ser. No. 11/213,641, filed the sameday as the present application, the entire content of each of which isincorporated herein by reference.

TECHNICAL FIELD

The invention relates to computer networks and, more particularly, tovirtual private local area network service (VPLS) instances establishedover computer networks.

BACKGROUND

A computer network is a collection of interconnected computing devicesthat exchange data and share resources. In a packet-based network thecomputing devices communicate data by dividing the data into smallblocks called packets. Certain devices within the network, such asrouters, maintain routing information that describes routes through thenetwork. In this way, the packets may be individually routed across thenetwork from a source device to a destination device. The destinationdevice extracts the data from the packets and assembles the data intoits original form. Dividing the data into packets enables the sourcedevice to resend only those individual packets that may be lost duringtransmission.

Virtual private local area network service (VPLS) instances are oftenused to extend two or more remote customer networks, i.e., VPLS sites,through a public network, such as the Internet, as if the public networkdoes not exist. VPLS instances often transport layer two (L2)communications, such as Ethernet packets, between customer networks viathe public network. In a typical configuration, routers coupled to thecustomer networks define label switched paths (LSPs) within the publicnetwork to carry encapsulated L2 communications as if these customernetworks were directly attached to the same LAN.

In some cases, a VPLS multicast instance may be configured to carry L2multicast traffic, such as Internet Protocol Television (IPTV), desktopconferences, corporate broadcasts, music and video web casts, and otherforms of multimedia content. VPLS multicast instances typically rely oningress replication to transmit the multicast traffic from a multicastsource to subscriber devices within the customer networks. Ingressreplication causes an ingress router of a VPLS to replicate a multicastdata packet of a particular multicast group and send it to each egressrouter of the VPLS on the path to a subscriber device of that multicastgroup. However, ingress replication may be a reasonable model only whenthe bandwidth of the multicast traffic is low and/or the number ofreplications performed by the ingress router for a particular multicastdata packet is small.

In order to send multicast packets only to the egress routers of theVPLS that have subscriber devices for that traffic, the ingress routerof the VPLS may use internet group management protocol (IGMP) snoopingor protocol independent multicast (PIM) snooping between the routers andthe customer networks. However, each router in the network then has tomaintain state for all of the multicast source and group (S,G) entriesin each of the VPLS multicast instances to which the router belongs. Inaddition, the PIM snooping is also performed on pseudo-wire (PW)interfaces between the routers within the network. This introduces anon-negligible overhead on the routers.

SUMMARY

In general, principles of the invention relate to providing virtualprivate local area network service (VPLS) multicast instances across apublic network by utilizing multicast trees. For example, the VPLSmulticast instances may transport layer two (L2) multicast traffic, suchas Ethernet packets, between remote customer networks via the publicnetwork. The principles described herein enable VPLS multicast instancesto handle high-bandwidth multicast traffic. The principles may alsoreduce the state and the overhead of maintaining the state in thenetwork by removing the need to perform snooping between routers withinthe network.

For example, a router within a public network, such as the Internet,learns customer source-group (<C-S, C-G>) entries of other routers inthe public network without performing snooping on a backbone of thepublic network. A router may use IGMP/PIM (internet group managementprotocol/protocol independent multicast) snooping to learn the <C-S,C-G> entries of the local router. In addition, a router may use reliabletransport, such as BGP or PIM with reliability extensions, to transmitcontrol messages, such as customer join/prune messages, between remoterouters within the public network.

Multicast trees may be setup across the public network using protocolindependent multicast (PIM) or non-PIM protocols, such as multi-protocollabel switching (MPLS) protocols. The MPLS protocol may include thelabel distribution protocol (LDP) and the resource reservation protocol(RSVP), which may be extended to include traffic engineering (TE)capabilities. The multicast trees may comprise aggregate multicast treesthat support more than one VPLS multicast instance. In addition, datamulticast trees may be setup to transmit traffic for specifichigh-bandwidth multicast groups. The multicast trees may be source treesor shared trees.

In one embodiment, a method comprises establishing a multicast treehaving a source device and one or more destination devices within anetwork, wherein each of the one or more destination devices belongs toat least one VPLS multicast instance. The method further comprisestransmitting multicast data packets for the VPLS multicast instancesfrom the source device to the one or more destination devices on themulticast tree.

In another embodiment, a network device comprises a control unit thatestablishes a multicast tree having a source device and one or moredestination devices within a network, wherein each of the destinationdevices belongs to at least one VPLS multicast instance. The controlunit also transmits multicast data packets for the VPLS multicastinstances from the source device to the one or more destination deviceson the multicast tree.

In another embodiment, a computer-readable medium comprises instructionsthat cause a programmable processor to establishing a multicast treehaving a source device and one or more destination devices within anetwork, wherein each of the one or more destination devices belongs toat least one VPLS multicast instance. The instructions further cause theprogrammable processor to transmit multicast data packets for the VPLSmulticast instances from the source device to the one or moredestination devices on the multicast tree.

In a further embodiment, a system comprises a source device within anetwork, one or more destination devices within the network, whereineach of the one or more destination devices belongs to at least one VPLSmulticast instance, and a multicast tree established within the network,wherein the multicast tree transmits multicast data packets for the VPLSmulticast instances from the source device to the one or moredestination devices.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example service provider (SP)network in which provider edge (PE) routers support at least one virtualprivate local area network service (VPLS) multicast instance.

FIG. 2 is a block diagram illustrating an exemplary PE router capable ofsupporting one or more VPLS multicast interfaces.

FIG. 3 illustrates an exemplary BGP encoding of network layerreachability information (NLRI).

FIG. 4 illustrates an example tree identifier (TI) attribute.

FIG. 5 is a flowchart illustrating an example procedure for setting upan aggregate default tree between a root of the aggregate default treeand an egress PE router.

FIG. 6 is a flowchart illustrating an example procedure for setting upan aggregate data tree between a root of the aggregate data tree and anegress PE router.

FIGS. 7A and 7B are flowcharts illustrating two exemplary processes ofswitching from an aggregate default tree to an aggregate data tree.

FIG. 8 illustrates an example IP encapsulated packet for transmission ona PIM-based multicast tree.

FIG. 9 illustrates an example MPLS encapsulated packet for transmissionon a RSVP- or LDP-based multicast tree.

FIG. 10 is a flowchart illustrating an example process of forwardingmulticast data packets on an aggregate multicast tree across a publicnetwork.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an example service provider (SP)network 10 in which provider edge (PE) routers 12A-12C (“PE routers 12”)support at least one virtual private local area network service (VPLS)multicast instance. In the illustrated embodiment, PE router 12A sets upa multicast tree 15 across the SP network 10 to provide layer two (L2)multicast service between PE routers 12. For example, multicast tree 15may transport L2 multicast traffic from a multicast source 24 tosubscriber devices within at least one of the VPLS A site and the VPLS Bsite coupled to PE routers 12. In other embodiments, multicast tree 15may be setup by any one of PE routers 12.

SP network 10 may comprise the Internet or another public network. Insome cases, SP network 10 may comprise a multi-protocol label switching(MPLS) network. Each of the VPLS sites may include a local area network(LAN) or a wide area network (WAN) that comprises a plurality ofsubscriber devices, such as desktop computers, laptops, workstations,PDAs, wireless devices, network-ready appliances, file servers, printservers or other devices.

Each of PE routers 12 couples to one or more of the VPLS sites viacustomer edge (CE) routers 16A-16E (“CE routers 16”). For example, PErouter 12A is coupled to VPLS A site 18A and VPLS B site 18B via CErouter 16A and CE router 16B, respectively. PE router 12A is alsocoupled to multicast source 24. PE router 12B is coupled to VPLS A site20 via CE router 16C. PE router 13C is coupled to VPLS A site 22A andVPLS B site 22B via CE router 16D and CE router 16E, respectively.Multicast tree 15 couples PE routers 12 to each other via provider (P)routers 14A-14D (“P routers 14”) within SP network 10.

In the illustrated embodiment, VPLS A and VPLS B established across SPnetwork 10 are capable of carrying high-bandwidth multicast traffic withmulticast trees. For example, VPLS A and VPLS B may carry L2 multicasttraffic, such as Internet Protocol Television (IPTV), desktopconferences, corporate broadcasts, music and video web casts, and otherforms of multimedia content, from multicast source 24 to subscriberdevices within the VPLS A sites and the VPLS B sites. Principlesdescribed herein may reduce the state and the overhead of maintainingthe state in SP network 10 by removing the need to perform snoopingbetween PE routers 12 within SP network 10.

In this example, each of PE routers 12 includes a virtual switchinterface (VSI) (not shown) for each VPLS multicast interface to whichit has membership. PE routers 12 advertise their VPLS membership, i.e.,the VSIs configured for multicast, to the other PE routers 12 using theborder gateway protocol (BGP) or another VPLS auto-discovery mechanism.In this way, each of PE routers 12 in SP network 10 have a complete viewof the VPLS memberships of the other PE routers.

Each of PE routers 12 may also discover customer source-group (<C-S,C-G>) entries for the other PE routers 12 within SP network 10. Learningthe <C-S, C-G> entries allows PE routers 12 to send multicast packet forspecific <C-S, C-G> entries only to the other PE routers 12 that havesubscriber devices of the specific multicast group. This substantiallyeliminates flooding of PE router 12 that do not have subscriber devicesof the specific multicast traffic.

For example, PE router 12A may learn the <C-S, C-G> entries from thelocal VSIs included on PE router 12A by performing IGMP/PIM (internetgroup management protocol/protocol independent multicast) snooping.Snooping is used because there is no PIM adjacency between CE routers16A, 16B and PE router 12A. IGMP/PIM snooping allows PE router 12A tobuild a database of customer join messages sent by subscriber deviceswithin VPLS sites 18A and 18B.

In conventional VPLS multicast instances that use ingress replication, aPE router may use PIM snooping to learn <C-S, C-G> entries of remoteVSIs included on other PE routers in the network. However PIM snoopingis computationally expensive. Furthermore the periodic nature of PIMJoin/Prune messages implies that snooping PIM messages places even agreater processing burden on a PE router.

As described herein, PE routers 12 may use a reliable transport protocolto transmit customer control messages between PE routers 12 within SPnetwork 10. A reliable transport protocol, such as BGP or PIM extendedto include a refresh reduction mechanism, substantially eliminates theneed to perform PIM snooping between PE routers 12 within SP network 10.For example, PE router 12A converts snooped customer join/prune messagesfrom local VPLS sites 18A and 18B to reliable protocol messages. PErouter 12A uses BGP or PIM with reliability extensions to transmit theconverted customer join/prune messages to the other PE routers 12 in SPnetwork 10.

Each of PE routers 12 maintains a database of <C-S, C-G> entries thatare snooped from the local PE router and discovered from the remote PErouters for each VPLS multicast instance. In some cases, PE routers 12may transmit customer control traffic only to PE router 12A, which iscoupled to multicast source 24. However, PE routers 12B and 12C do nothave routes through SP network 10 to reach multicast source 24.Therefore, the customer join/prune messages are sent to all of PErouters 12 that belong to the particular VPLS. The PE router may useeither PIM or BGP to transmit the customer control traffic.

As described above, VPLS auto-discovery allows each of PE routers 12 tolearn the VPLS memberships of the other PE routers 12 within SP network10. In the case of PIM, one of PE router 12 sends the customer controlmessages to all of PE routers 12 that belong to the VPLS using unicastPIM messages. In order to send the customer control messages to aparticular remote PE router, e.g., PE router 12A, the control messagesare encapsulated in the pseudo-wire (PW) used to reach PE router 12A.

Although join message suppression is disabled and PIM refresh reductionmechanisms are used, the use of PIM for propagation of customer controlinformation may include scalability limitations. BGP, on the other hand,includes route-reflector machinery that allows PE routers 12 to transmitcustomer control traffic with increased scalability.

PE router 12A may setup multicast tree 15 across SP network 10 totransport customer multicast data with one of a variety of tunnelingtechnologies. For example, multicast tree 15 may be setup by PE router12A using PIM or non-PIM protocols, such as MPLS protocols. The MPLSprotocols include the label distribution protocol (LDP) and the resourcereservation protocol (RSVP), which may be extended to include trafficengineering (TE) capabilities. In the case of PE router 12A usingRSVP-TE, multicast tree 15 may comprise a point-to-multipoint (P2MP)label switched path (LSP).

In the illustrated embodiment, multicast tree 15 comprises an“aggregate” multicast tree capable of transmitting traffic for both VPLSA and VPLS B across SP network 10. In this way, SP network 10 does notneed to separately maintain state per each VPLS as one multicast tree 15can be use to support multiple VPLS multicast instances. In some cases,multicast tree 15 may comprise an aggregate “default” tree mapped toVPLS A and VPLS B. In other cases, since PE router 12A is coupled tomulticast source 24, multicast tree 15 may comprise an aggregate “data”tree mapped to specific multicast groups. These embodiments are furtherdescribed below.

In the case where multicast tree 15 comprises an aggregate default tree,multicast tree 15 carries traffic of all the multicast groups requestedby subscriber devices within both VPLS A and VPLS B. PE router 12A maysetup multicast tree 15 as an aggregate default tree by using BGP todiscover egress PE routers 12B and 12C, i.e., the leaves of multicasttree 15. VPLS auto-discovery allows PE router 12A to learn the VPLSmembership information of PE routers 12B and 12C. This in turn allowsthe creation of the aggregate default tree mapped to VPLS A and VPLS B.The leaves of the aggregate default tree are the PE routers within SPnetwork 10 that belong to one or more of the VPLS multicast instancesmapped to the aggregate default tree. In other embodiments, aggregatedefault tree 15 maybe set up by any of PE routers 12 or by a rendezvouspoint (RP), e.g., one of P routers 14, within SP network 10.

By removing the need to separately maintain per VPLS state in SP network10, aggregate default trees may effectively reduce the number of treesin SP network 10 and the signaling overhead associated with maintainingthese trees. However, since aggregate default tree 15 carries trafficfor all the multicast groups requested in both VPLS A and VPLS B,aggregate default tree 15 may deliver a multicast data packet for aparticular group to some of PE routers 12 that do not have subscriberdevices for that multicast group.

In the case where multicast tree 15 comprises an aggregate data tree,multicast tree 15 only carries traffic of specific multicast groups frommulticast source 24 to the VPLS sites that include subscriber devices ofthe multicast traffic. Multicast tree 15 may be setup as an aggregatedata tree 15 by a router in SP network 10 that is coupled to multicastsource 24, i.e., PE router 12A. PE router 12A may setup multicast tree15 as an aggregate data tree by using a reliable transport protocol todiscover egress PE routers 12B and 12C, i.e., the leaves of multicasttree 15. In this way, PE router 12A is able to create a separatemulticast tree 15 as an aggregate data tree for specific, high-bandwidthmulticast groups. More than one multicast group may be mapped onto theaggregate data tree. In addition, the multicast groups mapped to theaggregate data tree may also belong to different VPLS multicastinstances.

As an aggregate data tree, multicast tree 15 transmits the traffic forthese multicast groups only to those PE routers 12 with subscriberdevices of the specific multicast groups. This avoids flooding other PErouters in the VPLS that have not requested the specific multicasttraffic. When router 12A receives multicast traffic of one of thespecific multicast groups mapped to multicast tree 15, PE router 12A mayswitch from an aggregate default tree to an aggregate data tree, e.g.,multicast tree 15, to transmit the multicast traffic.

In addition, multicast tree 15 can be either a “source” tree or a“shared” tree. As used herein, a source tree is used to carry trafficonly for the multicast VSIs that exist locally on the root of the tree.For example, in the case where PE router 12B is the root of multicasttree 15, as a source tree, multicast tree 15 may only carry traffic forVPLS A to which PE router 12B belongs. In contrast, a shared tree cancarry traffic belonging to VSIs that exist on other PEs as well. Forexample, in the case where PE router 12B is the root of multicast tree15, as a shared tree, multicast tree 15 may carry traffic for VPLS A towhich PE router 12B belongs and for VPLS B to which PE router 12B doesnot belong.

FIG. 2 is a block diagram illustrating an exemplary PE router 30 capableof supporting one or more VPLS multicast interfaces in accordance withthe techniques described herein. As one example, PE router 30 maycomprise an ingress router or root of a multicast tree establishedacross a public network, such as the Internet. PE router 30 may alsocomprise an egress router or leaf of a multicast tree established acrossthe public network by another PE router. PE router 30 may operatesubstantially similar to any of PE routers 12 from FIG. 1.

In this example, PE router 30 includes interface cards 32A-32N (“IFCs32”) that receive multicast packets via inbound links 33A-33N (“inboundlinks 33”) and send multicast packets via outbound links 34A-34N(“outbound links 34”). IFCs 32 are typically coupled to links 33, 34 viaa number of interface ports. Router 30 also includes a control unit 31that determines routes of received packets and forwards the packetsaccordingly via IFCs 32.

A system administrator may specify configuration information for PErouter 30 via a user interface 44 included within control unit 31. Theconfiguration information may then be stored in database (DB) 45 coupledto user interface 44. User interface 44 may include a display, akeyboard, a mouse or another type of input device.

Control unit 31 maintains routing information 46 that describes thetopology of a network and, in particular, routes through the network.Routing information 46 may include, for example, route data thatdescribes various routes within the network, and corresponding next hopdata indicating appropriate neighboring devices within the network foreach of the routes. Router 30 updates routing information 46 toaccurately reflect the topology of the network.

Control unit 31 also maintains forwarding information 47 that associatesnetwork destinations with specific next hops and corresponding interfaceports. In general, when router 30 receives a multicast packet via one ofinbound links 33, control unit 31 determines a destination andassociated next hop for the packet in accordance with routinginformation 46 and forwards the packet on one of outbound links 34 tothe corresponding next hop in accordance with forwarding information 47based on the destination of the packet.

Control unit 31 provides an operating environment for protocols 36 toexecute. In the illustrated embodiment, protocols 36 include RSVP 38,BGP 39, PIM 40, IGMP 41, and LDP 42. Control unit 31 also includesauto-discovery module 48, binding module 49, aggregation module 50,default tree setup module 51, data tree setup module 52, multicast treeinterfaces 54, PE-PE exchange module 56, and PE-CE snooping module 58.In other embodiments, binding module 49 and aggregation module 50 maycomprise sub-modules within auto-discovery module 48. In still otherembodiments, aggregation module 50 may comprise a sub-module in defaulttree setup module 51 and/or data tree setup module 52.

Auto-discovery module 48 advertises the VPLS multicast memberships of PErouter 30 to other PE routers in the network using BGP 39 or anotherVPLS auto-discovery protocol. Auto-discovery module 58 also receivesVPLS advertisements from the other PE routers. Therefore, PE router 30may have a complete view of the VPLS multicast memberships of the otherPE routers in the network. Auto-discovery module 48 then determineswhich PE routers in the network belong to the same VPLS multicastinstances as PE router 30. In some cases, auto-discovery module 48maintains PIM neighbor adjacencies with the PE routers of each of theVPLS multicast instances as long as the BGP advertisement is notwithdrawn. In this way, PE router 30 does not have to perform PIMneighbor adjacency management.

PE-CE snooping module 58 snoops multicast control messages between PErouter 30 and local CE routers of VPLS sites that include subscriberdevices of multicast traffic. For example, PE-CE snooping module 58 mayuse PIM 40 or IGMP 41 to snoop customer join/prune messages formulticast groups from subscriber devices within the local VPLS sites. PErouter 30 uses snooping to discover customer control information asthere is no PIM neighbor adjacency between PE router 30 and the local CErouters.

PE-PE exchange module 56 utilizes a reliable transport protocol totransmit customer control messages between PE router 30 and remote PErouters in the network. PE-PE exchange module 56 may use either BGP 39or PIM 40 extended to include a refresh reduction mechanism. In thisway, PE-PE exchange module 56 substantially eliminates the need toperiodically refresh customer control messages, such as customerjoin/prune messages. More specifically, PE-PE exchange module 56converts the customer control messages snooped by PE-CE snooping module58 to the reliable transport protocol. PE-PE exchange module 56 may thentransmit the customer control messages to the other PE routers withinthe public network without performing snooping between the remote PErouters.

PE router 30 supports various multicast data packet tunnelingtechnologies. Default tree setup module 51 and data tree setup module 52do not place any restrictions on the multicast technology used to setupmulticast trees across the network. For example, tree setup modules 51,52 may use RSVP 38, PIM 40, or LDP 42 to establish multicast trees. Insome cases, RSVP 38 may be extended to provide TE capabilities.

For example, default tree setup module 51 may use RSVP 38 to instantiatea P2MP LSP as a multicast tree. As described above, auto-discoverymodule 48 discovers the VPLS memberships of other PE routers in thenetwork. Once the leaves of the multicast default tree are discovered,default tree setup module 51 signals the LSP with conventional RSVP-TEP2MP procedures. Aggregation module 50 may then decide which of the VPLSmulticast instances to aggregate into a single multicast default tree.Binding module 49 maps the chosen VPLS multicast instances to theaggregate default tree and uses BGP 39 to advertise the mapping to theegress PE routers, or leaves, of the aggregate default tree.

As another example, default tree setup module 51 may use PIM 40 to setupa multicast tree in the core of the network. In this case, the aggregatedefault tree is termed an aggregate multicast distribution tree (MDT).Auto-discovery module 48 discovers the VPLS memberships of other PErouters in the network. Aggregation module 50 may then decide which ofthe VPLS multicast instances to aggregate into a single defaultmulticast tree. Binding module 49 maps the chosen VPLS multicastinstances to the aggregate MDT and uses BGP 39 to advertise the mappingto the egress PE routers, or leaves, of the aggregate MDT. The egress PErouters can then join the aggregate MDT. The egress PE routers also jointhe provider group address corresponding to the aggregate MDT.

In either case, the aggregate default tree may comprise either a sourcetree or a shared tree. In the case of a shared tree, the aggregatedefault tree can carry traffic that belonging to locally located VSIs ofPE router 30 and remotely located VSIs that exist on other PEs withinthe network. The other PEs in the network then tunnel the multicast datatraffic to the root of the shared tree, e.g., PE router 30, to betransmitted on the shared tree. In this way, the shared treesubstantially eliminates the need for each of the PE routers in thenetwork to establish an individual aggregate default tree.

When PE router 30 is coupled to a multicast source, data tree setupmodule 52 may establish an aggregate data tree across the network. Anaggregate default tree, by definition, maps to all the <C-S, C-G>entries belonging to all the VPLS multicast instances associated withthe aggregate default tree. An aggregate data tree maps to the specific<C-S, C-G> entries associated with subscriber devices coupled to theaggregate data tree. As one example, aggregate data trees may be used totransport high-bandwidth multicast traffic of one or more specificmulticast groups across the network. The specific multicast groups maybelong to multiple VPLS multicast instances. Aggregate data trees maysubstantially eliminate flooding of PE routers that do not havesubscriber devices for the specific high-bandwidth multicast traffic.

Prior to setting up aggregate data trees with data tree setup module 52,PE-CE snooping module 58 and PE-PE exchange module 56 learn which PErouters in the network that have subscriber devices of specificmulticast groups. PE-CE snooping module 58 learns the <C-S, C-G> entriesrequested by the local PE router 30. PE-PE exchange module 56 learns the<C-S, C-G> entries requested by remote PE routers in the network.Aggregation module 50 may then decide which of the multicast groups toaggregate into a single multicast data tree. Binding module 49 maps thechosen <C-S, C-G> entries to the aggregate data tree and uses BGP 39 toadvertise the mapping to the egress PE routers, or leaves, of theaggregate data tree. In the case where data tree setup module 52 usesPIM 40 to setup an aggregate data tree in the network, the aggregatedata tree is termed an aggregate data MDT.

Aggregate data tree creation may be triggered on criteria other thanbandwidth once customer join suppression is disabled. For example, therecould be a “pseudo wasted bandwidth” criteria such that PE router 30switches to an aggregate data tree when the bandwidth multiplied by thenumber of PE routers without subscriber devices for a specific multicaststream is above a specified threshold. This criterion may reduce theamount of bandwidth wasted by sparsely subscribed low-bandwidth groups.In addition, it may substantially eliminate the use of aggregate datatrees for a high-bandwidth multicast stream for which all the PE routersin the network have subscriber devices.

For either aggregate default trees or aggregate data trees, onceauto-discovery module 48 or PE-PE exchange module 56 has discovered theegress PE routers, i.e., leaves, of the multicast tree within thenetwork, aggregation module 50 determines which VPLS multicast instancesor <C-S, C-G> entries to aggregate into a single multicast tree. Theheuristics used to decide which VPLS multicast instances or <C-S, C-G>entries to aggregate may be implementation dependent. In some cases, PErouter 30 may use offline tools to aide in the aggregation decision.

The “congruency” of aggregation is defined by the amount of overlap inthe egress PE routers, or leaves, of the multicast trees that areaggregated. For example, the congruency of aggregate default treesdepends on the amount of overlap in memberships of VPLS multicastinstances that are mapped to the aggregate default tree. If there iscomplete overlap, aggregation is substantially perfectly congruent. Asthe overlap between the VPLS multicast instances that are mapped to theaggregate default tree reduces, the congruency reduces.

If aggregation module 50 performs aggregation that it is notsubstantially perfectly congruent, a PE router in the network mayreceive multicast traffic for VPLS multicast instances to which it doesnot belong. As the amount of multicast traffic for these unwanted VPLSmulticast instances increases, aggregation becomes less optimal withrespect to delivered traffic. Hence there is a tradeoff between reducingstate in the network and delivering unwanted traffic.

Aggregation module 50 may provide control over the congruency ofaggregation. For example, user interface 44 may receive aggregationconfiguration information from a system administrator. In this way, aservice provider may deploy aggregation depending on the VPLS membershipand traffic profiles in the network. The service provider may alsoengineer the maximum amount of unwanted VPLS multicast instances forwhich a particular PE router may receive traffic.

Aggregate default trees and aggregate data trees require a mechanism forthe egress PE routers to demultiplex the multicast traffic received overthe multicast trees. Since multicast traffic belonging to multiple VPLSmulticast instances can be carried over the same multicast tree, thereis a need to identify the VPLS to which the multicast packet belongs. Aningress router of the multicast tree may assign an inner label thatcorresponds to the multicast VSI for which the packet is intended. Theingress router uses this inner label while encapsulating a customermulticast data packet. Each of the egress PE routers of the multicasttree is capable of associating this inner label with the same VPLS andusing the inner label to demultiplex the multicast traffic received overthe aggregate default tree or the aggregate data tree.

For purposes of illustration, PE router 30 will be described as anegress PE router of the multicast tree. Using a downstream labelassignment would require all of the egress PE routers of the VPLS,including PE router 30, to agree on a common label for the VPLS.Therefore, the ingress PE router uses upstream label assignment toallocate the inner label. PE router 30 comprises a separate label spacefor every aggregate default tree and every aggregate data tree for whichPE router 30 is a leaf node. Control unit 31 creates a forwarding entrywithin forwarding information 47 for the inner label allocated by theingress PE.

When PE router 30 receives a packet over an aggregate multicast tree, anaggregate tree identifier (TI) specifies the label space in which toperform the inner label lookup. In some cases, control unit 31 maycreate a logical interface within multicast tree interfaces 54 thatcorresponds to the aggregate multicast tree. The logical interfacewithin multicast tree interface 54 then specifies the label space inwhich to perform the inner label lookup.

The ingress PE router informs the egress PE routers of the aggregatemulticast tree about the inner label as part of a discovery procedure.As described above, once a PE router sets up an aggregate default treeor an aggregate data tree, binding module 49 uses BGP 39 to announce theVPLS multicast instances or the <C-S, C-G> entries mapped to themulticast tree to the egress PE routers in the network. For an aggregatedefault tree, binding module 49 announces the mapping of all VPLSmulticast instances mapped to the aggregate default tree. Theannouncement also includes the inner label allocated by the ingress PEfor each VPLS and the aggregate default TI. For an aggregate data treebinding module 49 announces the mapping of all specific <C-S, C-G>entries mapped to the aggregate data tree. The announcement alsoincludes the inner label allocated by the ingress PE for each <C-S, C-G>entry and the aggregate data TI.

Control unit 31 may use IP/GRE (internet protocol/generic routingencapsulation) or MPLS to encapsulate multicast data packets fortransmission on aggregate default trees or aggregate data trees. If theaggregate default tree or the aggregate data tree uses MPLSencapsulation, the outer MPLS label and the incoming interface specifiesthe label space of the inner label. In this case,penultimate-hop-popping is disabled. If the aggregate default tree orthe aggregate data tree uses IP/GRE encapsulation, the root PE routersource address and the provider group address of the multicast treespecifies the label space of the inner label. A lookup in the labelspace of the inner label identifies the multicast VSI in which toperform the customer multicast lookup.

The architecture of router 30 illustrated in FIG. 2 is shown forexemplary purposes only. The invention is not limited to thisarchitecture. In other embodiments, router 30 may be configured in avariety of ways. In one embodiment, for example, some of thefunctionally of control unit 31 may be distributed within IFCs 32. Inanother embodiment, control unit 31 may include a routing engine thatperforms routing functions and maintains routing information base (RIB),e.g., routing information 46, and a forwarding engine that performspacket forwarding based on a forwarding information base (FIB), e.g.,forwarding information 47, generated in accordance with the RIB.

Control unit 31 may be implemented solely in software, or hardware, ormay be implemented as a combination of software, hardware, or firmware.For example, control unit 31 may include one or more processors whichexecute software instructions. In that case, the various softwaremodules of control unit 31 may comprise executable instructions storedon a computer-readable medium, such as computer memory or hard disk.

FIG. 3 illustrates an exemplary BGP encoding 60 of network layerreachability information (NLRI) that may be utilized as an extension toBGP to support embodiments of the invention. In this example, the NLRIis associated with a VPLS subsequent address family identifier (SAFI).BGP encoding 60 comprises at least a portion of a BGP advertisement. Asdescribed above, BGP advertisements are used for propagation of customermulticast control information, aggregate default tree discovery, andaggregate data tree discovery.

BGP encoding 60 encodes the NLRI into a length field 62, a MPLS labelfield 63, a route distinguisher (RD) field 64, a multicast source lengthfield 65, a multicast source field 66, and a multicast group field 67.Length field 62 comprises two octets and RD field 64 comprises eightoctets. The remaining fields within BGP encoding 60 comprise variablefield lengths.

For propagation of customer multicast control information, theinformation elements are included in BGP encoding 60. For a particularVPLS, the BGP advertisement includes the RD 64 configured for the VPLSmulticast instance. RD 64 uniquely identifies the <C-S, C-G> entry asthe PE router addresses could overlap between different VPLS multicastinstances. The BGP advertisement also includes the customer multicastsource address 66 and the customer multicast group address 67. Multicastaddresses 66 and 67 can be prefixes.

When the ingress PE router distributes this information, the BGPadvertisement includes a Route Target Extended Communities (RTEC)attribute. The RTEC attribute may be an “import route target” of eachVSI in the multicast tree. BGP distribution procedures ensure that theadvertised information gets associated with the right VSIs. The addressof the PE router that originates the customer control information iscarried in the BGP next-hop address of the MP_REACH_ATTRIBUTE.

The root of an aggregate default tree maps one or more VPLS multicastinstances to the aggregate default tree. For aggregate default treediscovery, the information elements for the VPLS multicast instancesthat are mapped to the aggregate default tree are included in BGPencoding 60 of the NLRI. RD field 64 is set to the configured RD for theVPLS and multicast group field 67 are set to zero. For a particularVPLS, the BGP advertisement includes the address of the root of theaggregate default tree and the inner label allocated by the root of theaggregate default tree for the VPLS.

When the ingress PE router distributes this information, the BGPadvertisement also includes an aggregate default tree identifier (TI)attribute and a RTEC attribute. The BGP next-hop address in the NEXT_HOPattribute or the MP_REACH_ATTRIBUTE may be set to the provider addressof the root of the aggregate default tree.

The root of an aggregate data tree maps one or more <C-S, C-G> entriesto the aggregate data tree. For aggregate data tree discovery, theinformation elements for the <C-S, C-G> entries that are mapped to theaggregate data tree are included in BGP encoding 60 of the NLRI. For aparticular <C-S, C-G> entry, the BGP advertisement includes the RD 64corresponding to the multicast enabled VSI. RD 64 uniquely identifiesthe <C-S, C-G> entry as the aggregate data tree root address couldoverlap between different VPLS multicast instances. The BGPadvertisement also includes the inner label allocated by the root of theaggregate data tree for the <C-S, C-G> entry. Furthermore, the BGPadvertisement includes the customer multicast source address 66 and thecustomer multicast group address 67. Multicast addresses 66 and 67 canbe prefixes in order to allow a range of customer source and groupaddresses to be mapped to the aggregate data tree.

When the ingress PE router distributes this information, the BGPadvertisement includes an aggregate data TI attribute and a RTECattribute. The address of the Aggregate Data Tree root is carried in theBGP next-hop address of the MP_REACH_ATTRIBUTE.

FIG. 4 illustrates an example tree identifier (TI) attribute 70 for usewith embodiments of the invention. TI attribute 70 enablesidentification of a specific aggregate default tree or aggregate datatree. As described above, aggregate default tree and aggregate data treeadvertisements carry TI attribute 70. TI attribute 70 includes whetherthe aggregate multicast tree is a shared aggregate multicast tree andthe type of tunneling protocol the root of the aggregate multicast treeused to establish the aggregate multicast tree. The TI attribute alsoincludes the identifier of the aggregate multicast tree based on thetree type.

TI attribute 70 includes an S bit field 72, a reserved field 73, a treetype field 74, and a tree identifier list field 75. S bit 72 is set whenthe aggregate multicast tree comprises a shared aggregate multicasttree. In other words, TI attribute 70 announces when the aggregatemulticast tree is capable of carrying traffic that belongs to VSIs thatdo not exist on the root of the aggregate multicast tree. Tree typefield 74 identifies the multicast tunneling technology used by the rootof the aggregate multicast tree to establish the aggregate multicasttree. In this way, tree type field 74 determines the semantics of treeidentifier list field 75.

Tree type field 74 may identify one of PIM-SSM (source specific mode)MDT, PIM-SM (sparse mode) MDT, or RSVP-TE P2MP LSP. When the type is setto PIM-SM MDT or PIM-SSM MDT, tree identifier list field 75 contains aPIM provider source-group (<P-S, P-G>) address. Hence MP_REACHidentifies a set of VPLS customer multicast trees, the TI attributeidentifies a particular aggregate multicast tree, and the BGPadvertisement of MP_REACH and TI creates a binding between the aggregatemulticast tree and the set of VPLS customer trees.

FIG. 5 is a flowchart illustrating an example procedure for setting upan aggregate default tree between a root of the aggregate default treeand an egress PE router. In the case of an aggregate default tree, theaggregate tree root may comprise a PE router or a RP within the network.The aggregate default tree may be a source tree or a shared tree. Theaggregate tree root and the egress PE router may operate substantiallysimilar to PE router 30 illustrated in FIG. 2.

The aggregate tree root uses BGP or another VPLS auto-discovery protocolto discover the VPLS memberships of the PE routers in the network (80).In this way, the aggregate tree root has a complete view of the VPLSmemberships of the other PE routers. From the complete VPLS listing, theaggregate tree root determines which of the VPLS multicast instances toaggregate into a single default multicast tree. The aggregate tree rootthen maps the aggregate default tree to these VPLS multicast instances(81). The aggregate tree root uses BGP to advertise the mappinginformation to the egress PE routers of the aggregate default tree (82).

At least one of the egress PE routers of the aggregate default treereceives the advertised mapping information (84). The egress PE routerexamines the mapping information and associates the aggregate defaulttree with one or more of the VPLS multicast instances to which theegress PE router belongs (85). The egress PE router then creates alogical interface to a label space corresponding to the aggregatedefault tree (86). As described herein, a multicast packet transmittedon the aggregate multicast tree includes an inner label that identifiesthe VPLS to which the packet belongs. The logical interface directs theegress PE router to the appropriate label space in which to perform aninner label lookup. The inner label lookup in turn determines the VSI inwhich the egress PE router performs a customer multicast packet lookup.

FIG. 6 is a flowchart illustrating an example procedure for setting upan aggregate data tree between a root of the aggregate data tree and anegress PE router. In the case of an aggregate data tree, the aggregatetree root comprises a PE router coupled to a multicast source. Theaggregate data tree may be a source tree or a shared tree. The aggregatetree root and the egress PE router may operate substantially similar toPE router 30 illustrated in FIG. 2.

The aggregate tree root uses customer join messages received on areliable transport protocol to discover the multicast group memberships,i.e., <C-S, C-G> entries, of the PE routers in the network (90). In thisway, the aggregate tree root has a complete view of the <C-S, C-G>entries of the other PE routers. From the complete multicast grouplisting, the aggregate tree root determines which of the <C-S, C-G>entries to aggregate into a single data multicast tree. The aggregatetree root then maps the aggregate data tree to these specific multicastgroups (91). The aggregate tree root uses BGP to advertise the mappinginformation to the egress PE routers of the aggregate data tree (92).

At least one of the egress PE routers of the aggregate data treereceives the advertised mapping information (94). The egress PE routerexamines the mapping information and associates the aggregate data treewith one or more of the specific <C-S, C-G> entries to which the egressPE router belongs (95). The egress PE router then creates a logicalinterface to a label space corresponding to the aggregate data tree(96). As described herein, a multicast packet transmitted on theaggregate multicast tree includes an inner label that identifies themulticast group to which the packet belongs. The logical interfacedirects the egress PE router to the appropriate label space in which toperform an inner label lookup. The inner label lookup in turn determinesthe VSI in which the egress PE router performs a customer multicastpacket lookup.

FIGS. 7A and 7B are flowcharts illustrating two exemplary processes ofswitching from an aggregate default tree to an aggregate data tree.Aggregate data trees provide a PE router with the ability to createseparate multicast trees for specific <C-S, C-G> entries. In some cases,aggregate data trees may be setup when an amount of bandwidth on anaggregate default tree is greater than a specified threshold. In othercases, aggregate data trees may be setup when the bandwidth multipliedby the number of PE routers without subscriber devices for a specificmulticast traffic is above a specified threshold. The ingress PE routerthat originates the aggregate data tree and the egress PE routers of theaggregate data tree switch to the aggregate data tree for the <C-S, C-G>entries that are mapped to the aggregate data tree.

FIG. 7A illustrates a first switching process. An ingress PE router setsup an aggregate data tree mapped to one or more multicast groups, i.e.,<C-S, C-G> entries (100). The ingress PE router then announces themapping of the <C-S, C-G> entries to the aggregate data tree to theegress PE routers of the aggregate data tree. Depending on the multicasttunneling technology, the ingress PE router may make the announcementbefore or after setting up the aggregate data tree. After the egress PErouters of the aggregate data tree receive the announcement, the egressPE routers setup forwarding entries, as described above, to receivemulticast traffic on the aggregate data tree.

Once the ingress PE router sets up the aggregate data tree, the ingressPE router sends multicast packets of the specific <C-S, C-G> entriesmapped to the aggregate data tree on both the aggregate data tree andthe aggregate default tree (101). After a preconfigured amount of time(yes branch of 102), the ingress PE router stops sending the multicastpackets of the specific <C-S, C-G> entries on the aggregate default tree(103).

FIG. 7B illustrates a second switching process. An ingress PE routersets up an aggregate data tree mapped to one or more multicast groups,i.e., <C-S, C-G> entries (105). The ingress PE router then announces themapping of the <C-S, C-G> entries to the aggregate data tree to theegress PE routers of the aggregate data tree. Depending on the multicasttunneling technology, the ingress PE router may make the announcementbefore or after setting up the aggregate data tree. After the egress PErouters of the aggregate data tree receive the announcement, the egressPE routers setup forwarding entries, as described above, to receivemulticast traffic on the aggregate data tree.

Once the ingress PE router sets up the aggregate data tree, the ingressPE router sends multicast packets of the specific <C-S, C-G> entriesmapped to the aggregate data tree on the aggregate default tree (106).After a preconfigured amount of time (yes branch of 107), the ingress PErouter stops sending the multicast packets of the specific <C-S, C-G>entries on the aggregate default tree (108). The ingress PE router thenstarts sending the multicast packets of the specific <C-S, C-G> entriesmapped to the aggregate data tree on the aggregate data tree (109)

FIG. 8 illustrates an example IP encapsulated packet 110 fortransmission on a PIM-based multicast tree. The example encapsulation ofFIG. 8 this may be used for any of the types of multicast treesdescribed herein, including aggregate default multicast trees, aggregatedata multicast trees, source multicast trees or shared multicast trees.

In this example, IP encapsulated packet 110 includes a provider IP(P-IP) header 112, a GRE header 113, an inner label 114, and anencapsulated payload 115. In the illustrated embodiments, encapsulatedpayload 115 includes a customer IP (C-IP) header 116 and a customerpayload (C-payload) 117. C-payload 117 may comprise a L2 multicast datapacket, such as an Ethernet packet, requested by a subscriber devicewithin a VPLS site.

P-IP header 112 contains the aggregate MDT or aggregate data MDTprovider group address as the destination address and the root addressof the MDT as the source address. The egress PE router of the MDT thatreceives IP encapsulated packet 110 performs a lookup on P-IP header 112and determines the forwarding entry or interface created within theegress PE router corresponding to the aggregate MDT or aggregate dataMDT. The forwarding entry or interface specifies the label space inwhich to perform a lookup of inner label 114.

Inner label 114 is unique within the context of the root of the MDT asit is assigned by the root of the MDT without coordination with other PErouters in the network. Therefore, inner label 114 is not unique acrossmultiple PE routers. In order to unambiguously identify a particularVPLS or multicast group, the egress PE router has to know inner label114 and the context within which inner label 114 is unique. The contextis provided by P-IP header 112.

The egress PE router strips P-IP header 112 and GRE header 113. Theegress PE router than performs the lookup of inner label 114 todetermine the VSI in which the egress PE router needs to perform thecustomer multicast data packet lookup. The egress PE router strips innerlabel 114 and sends the L2 multicast packet to the VSI for multicastdata forwarding.

FIG. 9 illustrates an example MPLS encapsulated packet 120 fortransmission on a RSVP- or LDP-based multicast tree. The exampleencapsulation of FIG. 9 this may be used for any of the types ofmulticast trees described herein, including aggregate default multicasttrees, aggregate data multicast trees, source multicast trees or sharedmulticast trees.

In this example, MPLS encapsulated packet 120 includes an MPLS label122, an inner label 123, and an encapsulated payload 124. In theillustrated embodiments, encapsulated payload 124 includes a customer IP(C-IP) header 125 and a customer payload (C-payload) 126. C-payload 126may comprise a L2 multicast data packet, such as an Ethernet packet,requested by a subscriber device within a VPLS site.

The egress PE router of an aggregate default tree or an aggregate datatree that receives MPLS encapsulated packet 120 performs a lookup onMPLS label 122 and determines the forwarding entry or interface createdwithin the egress PE router corresponding to the aggregate default treeor aggregate data tree. The forwarding entry or interface specifies thelabel space in which to perform a lookup of inner label 123.

Inner label 114 is unique within the context of the root of theaggregate tree as it is assigned by the root of the aggregate treewithout coordination with other PE routers in the network. Therefore,inner label 114 is not unique across multiple PE routers. In order tounambiguously identify a particular VPLS or multicast group, the egressPE router has to know inner label 123 and the context within which innerlabel 123 is unique. The context is provided by MPLS label 122.

The egress PE router strips MPLS label 122 and performs the lookup ofinner label 123 to determine the VSI in which the egress PE router needsto perform the customer multicast data packet lookup. The egress PErouter then strips inner label 123 and sends the L2 multicast packet tothe VSI for multicast data forwarding.

FIG. 10 is a flowchart illustrating an example process of forwardingmulticast data packets on an aggregate multicast tree across a publicnetwork. The process will be described herein in reference to SP network10 of FIG. 1. For purposes of illustration, multicast tree 15 comprisesan aggregate default tree. PE router 12A establishes multicast tree 15as a RSVP-TE P2MP LSP across SP network 10 between ingress PE router 12Aand egress PE routers 12B and 12C. PE router 12A maps VPLS A and VPLS Bto multicast tree 15. Multicast tree 15 may be a source tree or a sharedtree.

PE router 12A receives L2 multicast data packets, such as Ethernetpackets, for at least one of VPLS A and VPLS B from multicast source 24(130). PE router 12A encapsulates the multicast data packets for the oneof VPLS A and VPLS B that includes subscriber devices of the multicasttraffic (131). PE router 12A then transmits the encapsulated packet onmulticast tree 15, which is mapped to the VPLS (132).

Egress PE router 12C, for example, receives the encapsulated packet onmulticast tree 15 (134). Egress PE router 12C performs a lookup on theouter label of the encapsulated packet to determine the forwarding entrywithin egress PE router 12C that corresponds to multicast tree 15 (135).Egress PE router 12C then strips the outer label (136).

Egress PE router 12C performs a lookup on the inner label of theencapsulated packet to determine the VSI the corresponds to the VPLS(137). Egress PE router 12C then strips the inner label (138) and sendsthe multicast data packets to the corresponding VSI for forwarding tosubscriber devices of the multicast traffic within the VPLS site.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A method comprising: establishing a multicast tree comprising apoint-to-multipoint (P2MP) label switched path (LSP) forming a P2MPtunnel, the P2MP tunnel having a source device providing an ingress tothe P2MP tunnel and a plurality of destination devices providing aplurality of different egresses from the P2MP tunnel, wherein each ofthe destination devices belongs to at least one virtual private localarea network service (VPLS) multicast instance; and transmittingmulticast data packets for the VPLS multicast instances from the sourcedevice through the P2MP tunnel to the destination devices on themulticast tree, wherein establishing the multicast tree comprises:discovering, with the source device, the VPLS multicast instances ofwhich the destination devices are members; mapping more than one of thediscovered VPLS multicast instances to the multicast tree; allocating aninner label with the source device for each of the more than one VPLSmulticast instances mapped to the multicast tree; advertising mappinginformation from the source device to the one or more destinationdevices, wherein the mapping information specifies which of thediscovered VPLS multicast instances are mapped to the multicast tree;with the source device, including the allocated inner labels within themapping information advertised from the source device to the destinationdevices to allow the destination devices to demultiplex the multicastdata packets received over the multicast tree; in response to themapping information advertised by the source device, maintaining, withineach destination device, a separate label space for the multicast tree,wherein the label space within each destination device includesforwarding entries for the inner labels allocated by the source device,and wherein transmitting the multicast data packets from the sourcedevice comprises encapsulating each of the multicast data packets at thesource device with an outer label and one of the inner labels, whereinthe outer label identifies the label space within the destination devicethat corresponds to the multicast tree, and wherein the inner label isbased on the VPLS multicast instance with which the multicast datapacket is associated.
 2. The method of claim 1, wherein transmittingmulticast data packets comprises transmitting the multicast data packetsonly for local VPLS multicast instances of the source device.
 3. Themethod of claim 1, wherein transmitting multicast data packets comprisestransmitting the multicast data packets for both local and remote VPLSmulticast instances of the source device.
 4. The method of claim 1,wherein the network comprises a multi-protocol label switching (MPLS)network.
 5. The method of claim 1, wherein establishing the multicasttree comprises using a label switching protocol to establish themulticast tree, wherein the label switching protocol comprises one ofthe resource reservation protocol (RSVP) or the label distributionprotocol (LDP).
 6. The method of claim 1, wherein the multicast treecomprises one of a resource reservation protocol (RSVP) point tomulti-point (P2MP) label switched path (LSP), a label distributionprotocol (LDP) signaled P2MP LSP, or a protocol independent multicast(PIM) signaled P2MP LSP.
 7. The method of claim 1, further comprisingdetermining with the source device which of the discovered VPLSmulticast instances to aggregate onto the multicast tree.
 8. The methodof claim 1, further comprising: receiving, with one of the destinationdevices, the mapping information from the source device of the multicasttree; associating the multicast tree with one or more of the VPLSmulticast instances to which the destination device belongs; andcreating a forwarding entry in the destination device that correspondsto the multicast tree.
 9. The method of claim 8, further comprisingcreating a logical interface to the label space within the destinationdevice that corresponds to the multicast tree.
 10. The method of claim1, wherein transmitting multicast data packets comprises: receiving themulticast data packets from a multicast source; encapsulating themulticast data packets for the VPLS multicast instances that havedestination devices requesting the multicast data packets; andtransmitting the encapsulated packet for the VPLS multicast instances onthe multicast tree.
 11. The method of claim 1, further comprising:receiving the multicast data packets from the source device of themulticast tree; and sending the multicast data packets to a virtualswitch interface (VSI) of the destination device that corresponds to theVPLS for which the multicast data packets were intended based on aninner label allocated by the source device that identifies the VPLS. 12.The method of claim 1, wherein transmitting the multicast data packetscomprises transmitting multicast data packets of specific multicastgroups that belong to the VPLS multicast instances from the sourcedevice to the one or more destination devices on the multicast tree. 13.A network device comprising: a control unit that establishes a multicasttree comprising a label switched path (LSP) forming apoint-to-multipoint (P2MP) tunnel, the P2MP tunnel having a sourcedevice providing an ingress to the multicast tree and a plurality ofdestination devices providing a plurality of different egresses from themulticast tree, wherein each of the destination devices belongs to atleast one virtual private local area network service (VPLS) multicastinstance; an auto-discovery module that discovers the VPLS multicastinstances of which the destination devices are members; a binding modulethat maps more than one of the discovered VPLS multicast instances tothe multicast tree; and one or more output interfaces that transmitmulticast data packets for the VPLS multicast instances through the P2MPtunnel on the multicast tree, wherein the control unit allocates adifferent inner label for each of the more than one VPLS multicastinstances mapped to the multicast tree that identifies the VPLSmulticast instance and advertises mapping information to the one or moredestination devices, wherein the mapping information specifies which ofthe discovered VPLS multicast instances are mapped to the multicast treeand includes the allocated inner labels to allow the destination devicesto demultiplex the multicast data packets received over the multicasttree for the more than one VPLS multicast instances, and wherein thecontrol unit encapsulates each of the L3 multicast data packets with anouter label and one of the inner labels, wherein the outer labelidentifies a label space within the destination device that correspondsto the multicast tree.
 14. The network device of claim 13, wherein thecontrol unit transmits the multicast data packets only for local VPLSmulticast instances of the source device.
 15. The network device ofclaim 13, wherein the control unit transmits the multicast data packetsfor both local and remote VPLS multicast instances of the source device.16. The network device of claim 13, wherein the network comprises amulti-protocol label switching (MPLS) network.
 17. The network device ofclaim 13, wherein the label switching protocol comprises one of aresource reservation protocol (RSVP) or a label distribution protocol(LDP).
 18. The network device of claim 13, wherein the multicast treecomprises one of a resource reservation protocol (RSVP) point tomulti-point (P2MP) label switched path (LSP), a label distributionprotocol (LDP) signaled P2MP LSP, or a protocol independent multicast(PIM) signaled P2MP LSP.
 19. The network device of claim 13, furthercomprising: an auto-discovery module within the control unit thatdiscovers VPLS multicast instances of which of the plurality ofdestination devices are members; and a binding module within the controlunit that maps at least one of the discovered VPLS multicast instancesto the multicast tree and advertises mapping information to the one ormore destination devices.
 20. The network device of claim 19, furthercomprising an aggregation module that determines which of the discoveredVPLS multicast instances to aggregate onto the multicast tree.
 21. Thenetwork device of claim 13, wherein the control unit transmits multicastdata packets of specific multicast groups that belong to the VPLSmulticast instances from the source device to the one or moredestination devices on the multicast tree.
 22. A computer-readablemedium comprising instructions that cause a programmable processor to:discover, with a source device, a plurality of virtual private localarea network service (VPLS) multicast instances of which a plurality ofdestination devices are members; establish a multicast tree comprising alabel switched path (LSP) that forms a point-to-multipoint (P2MP)tunnel, the P2MP tunnel having a source device providing an ingress tothe multicast tree and a plurality of destination devices providing aplurality of different egresses from the multicast tree, wherein each ofthe one or more destination devices belongs to at least one of the VPLSmulticast instances; map more than one of the discovered VPLS multicastinstances to the multicast tree; allocate a different inner label withthe source device for each of the more than one VPLS multicast instancesmapped to the multicast tree; advertise mapping information from thesource device to the one or more destination devices, wherein themapping information specifies which of the discovered VPLS multicastinstances are mapped to the multicast tree by the source device; includethe allocated inner labels within the mapping information advertisedfrom the source device to the destination devices to allow thedestination devices to demultiplex the multicast data packets receivedover the multicast tree for the more than one VPLS multicast instances;encapsulate the multicast data packets at the source device with anouter label and one of the inner labels, wherein the outer labelidentifies the label space within the destination device thatcorresponds to the multicast tree, and wherein the inner label is basedon the MVPN with which the multicast data packet is associated; andtransmit the encapsulated multicast data packets for the VPLS multicastinstances from the source device through the P2MP tunnel to thedestination devices on the multicast tree.
 23. The computer-readablemedium of claim 22, wherein the instructions that cause the programmableprocessor to transmit multicast data packets cause the programmableprocessor to transmit the multicast data packets only for local VPLSmulticast instances of the source device.
 24. The computer-readablemedium of claim 22, wherein the instructions that cause the programmableprocessor to transmit multicast data packets cause the programmableprocessor to transmit the multicast data packets for both local andremote VPLS multicast instances of the source device.
 25. Thecomputer-readable medium of claim 22, wherein the instructions thatcause the programmable processor to transmit multicast data packetscause the programmable processor to transmit multicast data packets ofspecific multicast groups that belong to the VPLS multicast instancesfrom the source device to the one or more destination devices on themulticast tree.
 26. A system comprising: a source device within anetwork, wherein the source device provides an ingress to a point tomulti-point (P2MP) tunnel, the P2MP tunnel comprising a label switchedpath (LSP) that forms a multicast tree a tunnel for transmittingmulticast data packets for a plurality of different virtual privatelocal area network service (VPLS) multicast instance; one or moredestination devices within the network that provide egresses to the P2MPtunnel, wherein each of the one or more destination devices belongs toat least one of a plurality of VPLS multicast instances; wherein thesource device allocates a different inner label for each of theplurality of different VPLS multicast instances and communicates theallocated inner labels to the destination devices, wherein each of thedestination devices maintain a label space that includes forwardingentries for the inner labels allocated by the source device, wherein thesource device encapsulates each of the multicast data packets with anouter label and one of the inner labels, wherein the outer labelidentifies the label space within the receiving destination device thatcorresponds to the P2MP tunnel, and the inner label identifies the VPLSmulticast instance for which the multicast data packets are intended,and wherein the destination devices demultiplex the multicast datapackets received over the multicast tree for the plurality of differentVPLS multicast instances and select a virtual routing and forward (VRF)for forwarding the multicast data packets based on the different innerlabels of the multicast data packets.
 27. The system of claim 26,wherein the multicast tree transmits multicast data packets of specificmulticast groups that belong to the VPLS multicast instances from thesource device to the one or more destination devices on the multicasttree.